Semiconductor junction



Unite States Patent Q i SEMICONDUCTOR JUNCTION Kurt Lehovec, Williamstown, Mass., assignor t Sprague Electric Company, North Adams, Mass, a corporation of Massachusetts Application January 28, 1957, Serial No. 636,821

1 Claim. (Cl. 1148-15) This invention relates to semiconductor devices, and more particularly to surface alloying processes together with apparatus for producing alloy junctions and the junctions formed by the processes'and apparatus. This application is a continuation-in-part of Serial No. 516,180, filed June 17, 1955, now US. Letters Patent 2,779,877, issued January 29, 1957.

The more common forms of junction transistors utilize a semiconductor with three regions of conductivity, e.g. an N-region sandwiched between two P-regions (PNP) or one P-region sandwiched between two N-regions (NPN). A typical example of such a transistor is the germanium PNP alloy junction transistor which is produced by fusing indium on opposite surfaces of an N- type germanium wafer; the molten indium dissolves some germanium of the wafer, which is recrystallized as a P- type germanium layer after solidification of the indiumgermanium alloy. It should be understood that although this invention will be explained in terms of an indium germanium PNP alloy junction transistor, the scope of the invention is not limited to a PNP junction structure or to germanium as the semiconducting body. The invention applies as well to an NPN or other multiple junction device, and to other semiconducting materials such as silicon and the groups III-V intermetallic compounds.-

One of the more important properties of the PNP indium germanium alloy junction transistor is its frequency response, which depends largely on the width of the N- type germanium region between the two P-type regions. In order to extend the frequency response to higher frequencies, said width should be made as small as possible, a few tenths of a mil being a typical dimension.

In order to control the electrical properties of these alloy junction devices, the penetration of the rectifying electrode into the N-type germanium body must not be greater than a fraction of a mil, and the diameter of the electrode must be no wider than a few mils. Prior art methods of producing alloy junction devices of this type have included plating gallium or indium on the N-type germanium; and in another case, fusing a small wafer of indium saturated with germanium onto the N-type germanium. The methods suffer difficulties in attaching electrode contact lead wires to the plating or to the wafer.

It is an object of this invention to provide a method for producing alloy junctions having very small width electrodes and contact wires. A further object of this invention is the provision of an economical method for producing alloy junction transistors which avoids handling tiny metal pellets commonly used for the alloying process. A still further object of this invention is to provide a method of attaching electrode contacts by alloying to a layer which is quite thin.

The objects of this invention are achieved by effusing a liquid metal, or metal alloy, from a suitable orifice onto a surface of the semiconductor to provide an electrode and/ or a contact lead wire.

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More particularly the objects of this invention are achieved by melting the electrode material, for example an indium germanium alloy; depositing the molten material in the capillary of a tube, e.g., glass or carbon, which has suitable heating means to maintain the alloy in a molten state; and then by means of pressure, forcing the alloy into contact with the semiconductor surface. The alloy solidifies immediately at the surface. By controlled withdrawal of the tube, and with continued pressure, a filament can be drawn from the electrode.

Still more particularly, the objects of this invention are achieved by alloying the semiconductor with a liquid metal (or metal alloy) which efiuses from a small orifice of a container of said liquid metal. By maintaining proper temperatures of the semiconductor and of the liquid metal leaving the orifice of the container, only a thin layer of the semiconductor will be dissolved by the metal when the metal and the semiconductor are brought in contact. There are essentially two ways in which this can be achieved. In one case a droplet of metal is re moved from the orifice and is brought into contact with the semiconductor. Only a thin layer of semiconductor will be dissolved in the metal if the volume of the metal is quite small and if the temperature of the metal is kept low enough so that a small amount of the semiconductor already dissolved, saturates the metal. In the other method, the metal is still in contact with the bulk metal in the container when brought into contact with the semiconducor surface, and is solidified so rapidly that only a small amount of the semiconductor metal has time to dissolve.

The invention and the above noted and other features will be understood more clearly and fully from the following detailed description with reference to the accom panying drawing, in which:

Fig. l is a cross-sectional view of a semiconductor body, having opposed micro-alloy junctions with electrodes produced according to this invention, and a con ventional base electrode; and,

Fig. 2 is a cross-sectional View of an apparatus for practicing the process of this invention.

Referring to Fig. 1 of the drawing, the semiconductor 10 has one planar surface 13 and an opposed surface having a depression 12. Conventional etching techniques, e.g. jet etching, may be employed to produce depression 12 and to produce the desired interelectrode thickness. Semiconductor 10 is provided with a base or gate electrode 14 of a non-rectifying nature. Electrode 14 may be composed of solder, and may be provided with a contact wire 15. Micro-alloy junctions 17 and 19 are provided in opposed faces 13 and 12, respectively, and have contact leads l6 and 18 extending therefrom. Solely by way of example, semi-conductor 10 is of N-type germanium and the micro-junctions are obtained in accordance with this invention by fusing an indium-germanium alloy to the body 10.

The electrode contacts 16 and 1-8 and junctions 17 and 19 can be applied in the manner shown in Fig. 2. A quantity of molten electrode material 20 is held in a capillary tube 22 of carbon, glass or quartz, for example. An internal tube diameter of about 5 mils or less is particularly suitable. The molten electrode material, which can be an indium-germanium alloy, is readily drawn up in such a capillary as by applying suction through a conveniently connected side tube 24 near the upper end of the capillary. While the composition of the alloy may have indium-germanium ratios of from about 2:1 to about 40:1, parts by weight, optimum properties are obtained with a composition of 20 parts indium to 1 part germanium. The germanium is added to the melt to prevent the indium from dissolving too much ger- Patented July 7, 1959' 3 maniupa in the crys' tal. .'1:he amount of germanium which can be added to the alloy is limited that germanium raises ih' melting point and decreases the ductility. The top of the capillary can be covered by a plug, not shown, oncanbe sealed shut it desired ln order to ke ep the lebtrfi t hi f m.,$.Q,i. -fy ig h ih r p ll y i i hh h hdhd.h ai hhh a i hh ph d hhi: hha et e hhtih ihfl 2, ethf th e h or h'h e. t heh h h h eiv h as he' h e set- Ih t hnill ry hhhthih h the .9 .l n. 1. 9 th iial hhedhy Pl'eq hsih h-we ehd h or W h h htlh h a fthas tt Qfijhh. .h h h h eh q eih take. a lied-Pr s es hh hhe a by r h .iq hh 4.W ..e. 8l Wer the 9 th heh (.IQIWBII Q 199i d m a1 9b forced hhtahd hih-e ahwtht e h t ea l th h y vB k p.- ih the d etc ht ve l ut mhhrh h e as a exam leahhta.59 1C: elqwthe. 9 .o th molten electrode materiahthe contacting endwill solidify and be firmly afiiired to the surface of the body .10, with mi e i hs q 99 h? m u h, hht rr 'i hfiy h ih tih ri i m i hzeahh e lhw-t h hr u f Alt h wid r. fi KQQW i m F can he d the bestadhesiomwhich closely approximates a welded joint, will be formed when a small temperature difierence hpre t. .T m9l l h germanium i t alloy rhr s th melhh Phih hi. t lle 1 91 n hrh controls thetime neededtosolidify the junctions 17 and IQ inthe germanium body 10. In this way the depth of he ih ht h st yrh e sil hhh ph i e m f me ing point of the alloy 241 and time of contact between the alloyand body It has been determined thatcontact 2 t QsWQ YesrePrhdh h e n et hh 9 e m n m b hdhe h hh hl h ert h ihterelece trode spacing. In other words, since the amount 0t eh hhqh t eihhsti h my h ehsm hereadily reproduced,the spacing between the electrodes maybe readily controlled by va rying the depth of the etching of depression 12 in body 10. After the end of themolten stem has solidified, the capillarycan be slowly withdrawn hem hhhlidifi d joint or wit h Q 1 l qh ihh pr s fre t9 t e e e 24, and hhmo eh electrode material will be pulled out, gradually solidifying h tthehd s e d 9 hrhrid n elongated am n By using the above technique, an electrode contact having a contact area two .mils wide or even less, is readily provided on bodies of germanium, silicon or even on the surface of other materials such as indium wafers or '4 the like. The contacted surface of body 10 can have a melting point above or below that of the electrode mat'e'rial Z0. Variations iii assuming the depth of the junction may be obtained by heating and cooling semiconductor 10 during and/or after contact with the molten alloy 20.

Since the time during which the liquid alloy is kept in contact with the germanium is quite short, it is frequently desirable to assure good and uniform wetting of the germanium. In an embodiment of this invention wet- 'ting is achievedby plating the surface of the germanium with a metal filn'r, e.g'.', indium, which dissdlves inthe liquid metal alloy. Thus, it should be understood that the pr se s nd. pp atu .Q myri n e tiqn may a as. be employed to provide contact leads to iunctions formed by any of the conventional prior art methods.

As many apparently widely different embodiments of this invention may be made without departing from the pirit and weathered. ti qbe. u d too th t t invention is ngt limited to, the specific. embodiments hereofexceptas d efined iri the appended claim. w tismh hhn d. .A prqh ss. fa p hdhhih hn a e junction in. a semi =9 1 l 9I hpsl ..Q I. P I i p ovi n a res v r f. molten metal or metal alloy capable of changir 1g the con; dhs i t l ih hhx hhdh th bQdY w ppo i e yp when brought into solidsolution in the semiconductor body, forcing a small amount of the molten rnetal or 7 meta QY,. 1EQ .l$h? e rtoirt ha s te mlhhh nd qhht ti i ph t i the em ndu or hd with h ma .amhhhto the l n e lue' me a l h lhhshlv hat s h t h h h th sem hhndh tot hsl ih h hhmal amo o metal r metal .e h thee y th z th i id, phase on h Q aid h m. h.h oi the h en met hr ta -all y and said dissolved pqrtion ot the, semiconductor body, whereby a pn-junction is produced.

kel'eheli cijis Cited the tile of patent 

